Power chip set for a switching mode power supply having a device for providing a drive signal to a control unit upon startup

ABSTRACT

A power chip set for a switching mode power supply includes a high voltage chip and a control unit chip. The high voltage chip contains a switching power metal-oxide-semiconductor (MOS) transistor being turned on/off under control of an output signal from the control unit, and a junction field effect transistor (JFET) coupled between a drain of the switching power MOS transistor and a power terminal of the control unit to serve as a start up element for driving the control unit during initiation, in which the JFET has a negative threshold voltage and the absolute value thereof is equal to the voltage for driving the control unit. The JFET structure in the high voltage chip further includes a Zener diode for over voltage protection of the control unit. The high voltage chip further contains a current-sense power MOS transistor coupled with the drain of the switching power MOS transistor for detecting a drain current of the switching power MOS transistor. The chip set can be packaged into a power module.

FIELD OF THE INVENTION

The present invention relates to an integrated circuit (IC) of aswitching mode power supply (SMPS), and more particularly to a powerchip set for an SMPS.

BACKGROUND OF THE INVENTION

It is a principal concern of the cost to manufacture a power supply inselecting between power supply types to be used in particularapplication and the components selected to construct them. Sinceintegrated circuit technology has advanced such that a majority of thecomplex switching mode circuits can be integrated on a single chip,switching mode power supplies have become cost competitive with muchsimpler linear power supplies.

A prior art flyback power supply with a voltage regulator isschematically shown in FIG. 1. In a power supply circuit 10, a full-wavebridge rectifier 12 accepts an AC power from a set of power inputterminals 14, and a DC power is therefore supplied to a transformer 18in association with a filter capacitor 16. The transformer 18 comprisesa primary winding 20 and a pair of secondary windings 22 and 24. Acapacitor 28 is charged through the winding 22 and a diode 26 such thata power supply output voltage is provided at a set of output terminals30 of the power supply circuit 10. On the other hand, a capacitor 34 ischarged through the winding 24 and a diode 32 to provide a feedbackvoltage, which is delivered to a pulse width modulator (PWM) 40 througha voltage divider consisting of resistors 52 and 54. An end of thewinding 20 is connected to a high voltage switching transistor 38 in aregulator circuit 36, which is turned on/off under the control of asignal from the PWM 40. The control signal from the PWM 40 to a gate ofthe switching transistor 38 oscillates at a frequency with a duty cyclesuch that a preset output voltage is maintained between the set ofoutput terminals 30 of the power supply circuit 10.

A high voltage power start up transistor 42 is used to start up theregulator circuit 36. When power is turned on, the transistor 42 isturned on and the capacitor 34 is charged to provide a line 50 withinternal low voltage power. Once the voltage on the line 50 reaches athreshold voltage, the PWM 40 functions, and when the voltage on theline 50 reaches a specific threshold voltage, a transistor 46 is turnedon. As a result, the threshold voltage from a voltage source 48 isconnected to a gate of the transistor 42, and thus the transistor 42 isturned off. As shown in FIG. 1, a resistor 44 is connected to a drain ofthe transistor 42.

To overcome the inconveniences caused by the high voltage pins of a chipand the high voltage itself, Balakrishnan disclosed in U.S. Pat. No.5,014,178 a high voltage power start up transistor directly connected toa drain of a high voltage switching transistor, while in U.S. Pat. No.5,313,381 a feedback control signal and a bias supply voltage combinedat a single pin with a metal-oxide-semiconductor (MOS) transistor, suchas that proposed in U.S. Pat. No. 4,811,075 to Eklund. However,complicated circuits and expensive manufacturing processes are presentedin these prior arts. In addition, it is desired a further integrationand excellent performance for an SMPS circuit.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a power chipset for an SMPS.

Briefly, a chip set according to the present invention comprises a highvoltage chip and a control unit chip, in which the high voltage chipcontains a junction field effect transistor (JFET) connected to thecontrol unit and served as its power start up element. The JFET has anegative threshold voltage and its absolute value is equal to the valueof the voltage to start up the control unit.

It is one aspect of the present invention that the JFET further containsa Zener diode for over voltage protection to the power start upterminals of the control unit.

One advantage of the present invention resides in that the JFET ismanufactured in a process compatible with the power MOS, and thereforethe JFET and the high voltage switching power MOS transistor areintegrated on a single chip.

Another advantage of the present invention resides in that the chip setis constituted of the high voltage chip and the control unit chip, suchthat expensive IC manufacturing process is avoided and a smaller chiparea is obtained. The chip set can be packaged into a module.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will become apparent to those skilled in the art uponconsideration of the following description of the preferred embodimentsof the present invention taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic view showing a prior art flyback power supplycircuit;

FIG. 2 is a functional block diagram of a power module according to anembodiment of the present invention;

FIG. 3 is a circuit diagram of a high voltage chip according to theembodiment of the present invention; and

FIG. 4 is a cross sectional view of the high voltage chip according tothe embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment according to the present invention is shown in FIG. 2, inwhich a power module 56 for a SMPS comprises a chip set composed of acontrol unit chip 58 and a high voltage chip 60. The high voltage chip60 contains high voltage circuit elements, while all control circuitscontained in the control unit chip 58 being not applied with highvoltage.

Provided in the control unit circuit 58, a pulse width modulationcomparator 62 receives and compares two signals, i.e., a feedback signalFB and an oscillation signal OSCI from an oscillator 64, through its twoinput terminals respectively. As a result, an output oscillation signalCOMP with a selected oscillation frequency and duty cycle is generatedby the pulse width modulation comparator 62. The feedback signal FB isalso connected to an over voltage and under voltage detection andfeedback detection unit 66 whose output signal together with an outputsignal of a shutdown/auto restart unit 68, an output signal of theoscillator 64 and the output oscillation signal COMP from the pulsewidth modulation comparator 62 are inputted into an AND gate 70, fromwhich a resultant control signal CTRL by logic decision is transmittedto a gate driver 72 to control ON/OFF state of a switching transistor74. In addition to the switching transistor 74, the high voltage chip 60further contains a power start up unit 76 and a current detection andprotection unit 78 connected to the control unit circuit 58. Theswitching transistor 74 has a source connected to a system referencevoltage, i.e., ground, and a drain connected to an output terminal OUTof the chip set 56, which is connected to the end of the primary winding20 of the transformer 18 as shown in FIG. 1. When the SMPS of thepresent invention is turned on, a start up voltage power is providedthrough the power start up unit 76 to the control unit 58 for theinitiation of the control unit 58. On the other hand, the currentdetection and protection unit 78 monitors the drain current of theswitching transistor 74 and produces an output signal I_(sns) which isalso inputted into the AND gate 70 in the control unit 58 to be one ofthe decision factors of the control single CTRL such that a protectionfor the chip set 56 is further provided.

FIG. 3 shows a circuit diagram of the high voltage chip 60, in which apower MOS transistor 74 serves as the switching transistor being turnedon/off under the control of the control signal outputted from thecontrol unit 58. The source of the transistor 74 is grounded and thedrain thereof is connected to the output terminal OUT of the chip set,which will be connected to the primary winding of the transformer 18 asshown in FIG. 1. A junction FET 80 is used as the start up element forthe control unit 58, and is connected between the drain of thetransistor 74 and the control unit 58 in order to provide the start upvoltage power for driving the control unit 58 to be initiated. Thethreshold voltage of the junction FET 80 has a negative value, and theabsolute value thereof is equal to the value of the power voltage fordriving the control unit 58. When the power of the SMPS is turned on, astart up voltage is applied to the control unit 58 from the junction FET80 to initiate the control unit 58, and then the junction FET 80 isturned off. Another power MOS transistor 82 serves as a current sensetransistor to detect the drain current of the transistor 74, which has adrain connected to the drain of the transistor 74, a gate connected tothe gate of the transistor 74 and a source connected to the control unit58. Moreover, a Zener diode 84 for over voltage protection of thecontrol unit 58 is coupled to the power terminal of the control unit 58.

The structure of the high voltage chip 60 is shown in FIG. 4, in whichthe junction FET is manufactured in a process compatible with that ofMOS. In the chip 60, an N⁻ drift layer 88 is epitaxially grown on an N⁺substrate 86, and then three P− wells 90, 92 and 94 are formed. P− base91, N⁺ region 96 and P⁺ region 98 are formed on the well 90. P− base 93,N⁺ region 100 and P⁺ region 102 are formed on the well 92. P⁺ region 114and N⁺ region 116 are formed on the well 94. A planar gate 104 is formedbetween the N⁺ regions 96 and 100 above the drift layer 88, and adielectric layer 106 is formed between the gate 104 and the drift layer88. A surface layer of the P− base 91 and 93 between the N⁺ regions 96and 100 below the dielectric layer 106 is a channel controlled by thegate 104. Therefore, the substrate 86, the drift layer 88, the wells 90and 92, the regions 96-102, the gate 104, and the dielectric layer 106constitute the switching power MOS transistor 74 mentioned above. Asurface portion of the drift layer 88 between the wells 92 and 94 isformed with N⁺ region 108, and insulation layers 110 and 112 are formedon the drift layer 88 between the N⁺ region 108 and P⁺ regions 102 and114, respectively. Therefore, the substrate 86, the drift layer 88, thewells 92 and 94, the P⁺ regions 102 and 104, and the N⁺ region 108constitute the junction FET 80 mentioned above, in which the structureis also known as static induction transistor (SIT). In the transistor80, as shown in the figure, the Zener diode 84 comprises the drift layer88, the N⁺ region 108 and the P⁻ wells 92 and 94. A planar gate 118 isproduced on a surface among the P− base 95, the N⁺ regions 116 and thedrift layer 88, and a dielectric layer 120 is formed between the gate118 and the P− base 95. A surface of the P− base below the gate 118 is agate channel. Therefore, the substrate 86, the drift layer 88, the P⁺region 114, the N⁺ region 116, the well 94, the P− base 95, the gate 118and the dielectric layer 120 constitute the current-sense transistor 82mentioned above.

While the present invention has been described in conjunction withpreferred embodiments thereof, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and scopethereof as set forth in the appended claims.

What is claimed is:
 1. A power chip set for a switching mode powersupply, comprising: a control unit chip with a power terminal forinputting a driving voltage and a terminal connected to a systemreference voltage; and a high voltage chip, including: switching powerMOS transistor having a drain connected to an output terminal of thechip set, a source connected to the system reference voltage and a gateconnected to the control unit, the switching power MOS transistor beingturned on/off by a control signal outputted from the control unit; ajunction field effect transistor coupled between the drain of theswitching power MOS transistor and the power terminal of the controlunit, for providing a start up voltage power for driving the controlunit when the power chip set is turned on; and a current-sense power MOStransistor, coupled with the drain of the switching power MOStransistor, for detecting a drain current of the switching power MOStransistor.
 2. The chip set according to claim 1, wherein the junctionfield effect transistor has a negative threshold voltage and theabsolute value thereof is equal to the voltage for driving the controlunit.
 3. The chip set according to claim 1, wherein the junction fieldeffect transistor structure further comprises a Zener diode for overvoltage protection of the control unit.
 4. The chip set according toclaim 1, wherein the control unit chip and the high voltage chip arepackaged into a power module.